Don't allow clicking on "Setup New U2F Device" unless an authenticator app has been set up.

- Also change the help message to indicate that an authenticator app is
  now a prerequisite for U2F.

app/views/profiles/two_factor_auths/show.html.haml

@@ -51,9 +51,9 @@
     %p
       Use a hardware device to add the second factor of authentication.
     %p
-      As U2F devices are only supported by a few browsers, it's recommended that you set up a
-      two-factor authentication app as well as a U2F device so you'll always be able to log in
-      using an unsupported browser.
+      As U2F devices are only supported by a few browsers, we require that you set up a
+      two-factor authentication app before a U2F device. That way you'll always be able to
+      log in - even when you're using an unsupported browser.
   .col-lg-9
     %p
       - if @registration_key_handles.present?

app/views/u2f/_register.html.haml

@@ -4,11 +4,18 @@
   %p Your browser doesn't support U2F. Please use Google Chrome desktop (version 41 or newer).
 
 %script#js-register-u2f-setup{ type: "text/template" }
-  .row.append-bottom-10
-    .col-md-3
-      %a#js-setup-u2f-device.btn.btn-info{ href: 'javascript:void(0)' } Setup New U2F Device
-    .col-md-9
-      %p Your U2F device needs to be set up. Plug it in (if not already) and click the button on the left.
+  - if current_user.two_factor_otp_enabled?
+    .row.append-bottom-10
+      .col-md-3
+        %button#js-setup-u2f-device.btn.btn-info Setup New U2F Device
+      .col-md-9
+        %p Your U2F device needs to be set up. Plug it in (if not already) and click the button on the left.
+  - else
+    .row.append-bottom-10
+      .col-md-3
+        %button#js-setup-u2f-device.btn.btn-info{ disabled: true } Setup New U2F Device
+      .col-md-9
+        %p.text-warning You need to register a two-factor authentication app before you can set up a U2F device.
 
 %script#js-register-u2f-in-progress{ type: "text/template" }
   %p Trying to communicate with your device. Plug it in (if you haven't already) and press the button on the device now.